Method of making an electrical contact



May 23, 1961 H. E. SPOONER 2,984,393

METHOD OF MAKING AN ELECTRICAL CONTACT Filed July 15, 1958 X F|Gd| FlG.2j O O 0 2 E o o 11 o r o a w o 0 O O FIG.4:. 15 i ill-III" g FIG-.5

17 1 2 FIG.6

IN VEN TOR.

HOWARD E. SPOONER BY Kusz (JAM JWZ, Q

ATTORNEY United States Patent METHOD OF MAKING AN ELECTRICAL CONTACTHoward E. Spooner, Lincoln, R.I., assignor, by mesne assignments, toEngelhard Industries, Inc., Newark, N.J., a corporation of DelawareFiled July 15, 1958, Ser. No. 748,761

7 Claims. (Cl. 29-15555) The present invention deals with electricalcontact elements suitable for use in making and breaking electricalcircuits and to methods for manufacturing such elements.

Heretofore, electrical contacts for application with voltage regulators,relays, and the like, have been formed from base metal strips withprecious metal inserts, inlays, or rivets which serve as the contactpoints. The precious metals have excellent current carrying capacitiesdue to their low electrical resistance, and they are highly resistant totarnish and electrical erosion, whereby electrical contacts formed fromthese metals have low contact surface resistance even with small contactpressures, and they are capable of maintaining low electrical resistancein service for long periods of operation.

In the manufacture of electrical contacts, the known methods include thesolder flushing of a precious metal strip from which solder facedinserts and the like are blanked prior to incorporating theinsert into abase metal strip which is subsequently subjected to contact formingoperations. In the blanking of the inserts, there is considerable scrapproduced, and this scrap necessitates costly refining to recoverprecious metal free of solder.

The present invention provides for improvements in the methods of makingcontacts.

It is an object of this invention to provide improved electricalcontacts and to save precious metals in the manufacture thereof. It isanother object of the present invention to provide a method for themanufacture of electrical contacts with a minimum of manufacturingoperations. Other objects and advantages of this invention will becomeapparent from the description hereinafter following and the drawingsforming a part hereof, in which:

Figure l is a top view of a base metal strip used in fabricatingcontacts embodying the invention,

Figure 2 is a top view of a perforated base metal strip,

Figure 3 is a side elevation of an individual insert plug greatlyenlarged,

Figure 4 is an enlarged side elevation of a modified insert,

Figure 5 is a partly cross sectional and partly elevational view of theapparatus employed in manufacturing contact members according to theinvention,

Figure 6 is a partial face view of a die shown in Figure 5,

Figure 7 is an enlarged section of a contact element blanked and formedfrom a composite strip,

Figure 8 is a partly cross sectional and partly elevational view of amodified form of the apparatus of Figure 5.

Referring to the drawing, there is shown in Figure 1 a wide flat strip10 formed of any suitable electrically conductive base metal, e.g.copper, nickel, and so forth, or alloys thereof, for example, brass,bronze, nickelsilver, etc. The strip 10, according to Figure 2, isformed by a punch press which punches out a series of bores or apertures11, the shape, location, and number of the bores being dependent uponthe shape and construction Patented May 23, 1961 ice of the electricalcontact element that is to be manufac: tured. The contact element whichwas chosen for illustration purposes has three precious metal contactpoints and there are three bores 11 punched in the strip 10 for eachcontact element to be inserted therein.

A series of locating or centering holes 12 is also punched in the strip10 in a line parallel to an edge of the metal strip, the centering holesbeing spaced apart by a distance substantially equal to the length ofstrip needed to form one contact element.

In Figure 3 there is shown a flat laminated metal plug 13, the shape andsize of which is such that it will press fit into a bore 11. Thethickness of the plug 13 preferably approximates that of the metal strip10. The plug 13 is punched out from a metal strip 14 (Figure 5) composedof metal or metals having a lower melting point than the metal of strip10. The strip 14, as illustrated, is formed from a compound metal stockhaving a base or foundation a of an inexpensive metal, such as, forexample, copper, and a layer b of a precious metal as gold, silver,platinum, palladium, and alloys of these metals. The layer b may beelectroplated on the surface of the foundation a, or the layers of metalmay be integrally bonded together by direct weld with the application ofheat and pressure, or by brazing or soldering, all as is well known tothose skilled in the art. Preferably, the strip 14 consists of preciousmetal, preferably silver or alloys thereof, e.g. coin silver, from whicha disc 15, shown by Figure 4, is blanked.

The plug 13, shown in Figure 3 as a disc, is punched out from the metalstrip 14, as illustrated by Figure 5, by passing the strip between a die16 and punch 17, with the face a of the strip towards the die and itsprecious metal face b towards the punch. The die 16 is provided with arecess 18 around the rim of the die hole 19 shown in Figure 6.

With the strip 10 prepared as illustrated in Figure 2, the next step inthe manufacture of the electric contacts of this invention comprisesfixing the discs 13 into the bores 11. This may be accomplished bysupporting the die 16 upon a base 21 and passing the flat strip 10-between the base and the die so that the discs 13 when punched from thestrip 14 will pass through the die hole 19 and become seated in thebores 11 in the strip 10. In order to maintain the strip in properrelative position so that the bores 11 are aligned with the die hole 19,the die 16 is provided in its bottom surface with the recess 22 ofsubstantially the same depth and width as the strip 10 for carrying thestrip. Figure 5 shows the punch 17 in a position after having punchedout the disc 13 and before it has seated the disc into a bore 11. Aguide plate 23 for the punch 17 is preferably mounted above the strip14.

It is apparent that at this state the plug or insert members 13 aremechanically secured by press fitting in the apertures 11, Having thusmechanically seated the inserts in the apertures 11, the composite stripis passed through a critical bonding phase comprising continuouslypassing the composite strip through a preheating zone of a furnace orthe like, whereby the strip is preheated to a temperature below themelting point of the insert 13 and preferably to a temperatureapproaching the melting point of the insert or just below the meltingpoint of the insert, and continuously passing the said strip from saidpreheating zone through an insert melting zone, which is maintained at atemperature at or above the melting point of the insert for a period oftime sufficient to provide only a molten skin or liquid surface on saidinsert and maintaining said inserts under said temperature, e.g. bycontrolling the speed of the strip through the melting zone until thesurface tension of the liquid face is broken, whereby the said moltenmetal contacts the walls of the apertures 11 and forms an interfacialalloy of the metal of said base metal strip and the metal of saidinsert, and continuously passing said strip from said melting zonethrough a temperature zone having a temperature below the melting pointof the insert. Since only the surface, or the face, of the inserts arein a molten condition, the insert is retained in the aperture, and uponcooling, results in a metal to metal bond between the insert and thebase metal strip. After cooling, the bonded strip is subjected to theformation of a contact element therefrom, as hereinafter moreparticularly described.

Referring to Figure 8, I have found that the abovedescribed method ofinserting a contact plug 15 into a base metal strip may be furthersimplified in that the necessity for preforming holes or apertures 11 ina strip can be eliminated. Instead of using a base metal strip 10 withperforations 11, a solid metal strip 10 without such perforations 11, asillustrated by Figure l, or including only the centering holes .12, ispositioned below the die 16, for example, in the recess 22. The plug orinsert is blanked or punched from a strip 15a as illustrated. As thepunch 17 is advanced toward the strip 10 through die 16, the plug 15acts as an end of a punch and blanks a corresponding or similarly shapedportion out of the strip 13, leaving itself deposited in the openingformed in the strip 1%} by the blanked portion 24 which is deposited inthe depression 27 in the base 21, said depression 27 preferably having adepth sufficient to leave the plug 15 as an insert in said strip 10 whenthe blank '24 contacts the bottom of the depression 27.

The finished contacts 25, shown by Figure 7, are formed from theassembled strip by'a series of stamping, drawing, and cuttingoperations, the particular sequence of these operations being immaterialfor the purposes of this invention so long as projections 26 which serveas the contact points are drawn out with their precious metal surface onthe apex or outer surface of the projection.

It is thus apparent that this invention provides a method formanufacturing an improved electrical contact having longer wearingproperties and greater corrosion resistance exactly where needed andpermits of the most economical use of costly noble or precious metals oralloys of these metals, since only the very point of contact need bemade from these metals.

Also, since no solder is employed in bonding the plugs 13 to strip 1%,it is apparent that the scrap resulting from blanking operations may berecovered by ordinary melting procedure instead of costly refining,especially when the insert plugs are composed of precious metal, e.g.coin silver.

it is understood that this invention is not limited to the specificembodiments shown and described and that various deviations may be madetherefrom without departingfrom the spirit and scope of the appendedclaims.

What is claimed is:

1. The method of making an electrical contact strip comprising a basemetal strip including an insert member having a melting point lower thanthe base metal strip, said method comprising the steps of punching atleast one insert member from a metal strip, press fitting said insertmember into an aperture in the base metal strip, preheating the assemblyof the strip containing the insert to a temperature below the meltingpoint of the insert, continuously passing said preheated strip through aheating zone maintained at a temperature at least at the melting pointof said insert, the duration of said continuous passage of said insertthrough said heating zone being limited to a time sufiicient to formonly a molten surface film of metal on said insert and until the surfacetension of the molten film. broken to form a metal to metal bond betweenthe insert and base metal strip, continuously passing said assembly ofthe strip containing the insert from said heating zone through atemperature zone having a temperature below the melting point of saidinsert.

2. The method of making an electrical contact according to claim 1,comprising inserting said insert member into preformed holes in saidmetal strip.

3. The method of making an electrical contactaccording to claim 1,comprising punching said insert member from said metal strip andcontinuously advancing said insert member through a solid strip, wherebysaid insert displaces a correspondingly shaped section from said stripand is deposited into the aperture formed by the displaced section.

4. The method of making an electrical contact from a base metal stripincluding an insert member having a melting point lower than the basemetal strip, said method comprising the steps of punching at least oneinsert member from a metal strip, press fitting said insert member intoan aperture in the base metal strip, preheating the assembly of thestrip containing the insert to a temperature below the melting point ofthe insert, continuously passing said preheated strip through a heatingzone maintained at a temperature at least at the melting point of saidinsert, the duration of said continuous passage of said insert throughsaid heating zone being limited to a time sufiicient to form a moltensurface film of metal on said insert and until the surface tension ofthe molten film is broken to form a metal to metal bond between theinsert and. base metal strip, continuously passing said assembly of thestrip containing the insert from said heating zone through a temperaturezone having a temperature below the melting point of said insert,stamping a contact member from said composite strip, said contact memberincluding at least one insert.

5. The method of making an electrical contact according to claim 4,comprising forming a projection insaid stamped contact member, the apexof said projection comprising said insert, whereby an electrical contactis formed having the insert as a contact point.

6. The method of making an electrical contact accord ing to claim 4,wherein said insert is punched from a precious metal strip.

7. The method of making an electrical contact according to claim 4,wherein said insert is punched from a laminated strip comprising aprecious metal layer and a base metal layer.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Welding Handbook, third ed., American Welding Society, 33West 39th St., New York 18, N.Y., 1950, p. 504, column 1, lines 17-55.

